Such cryostats are used, for example, in infrared detectors and missile guidance systems which operate under cryogenic conditions. Typically, the cryostat includes a cryogenic cooler, such as a Joule Thomson heat exchanger, which is connected to a reservoir within which an inventory of liquid cryogen is collected that is derived from the initially cooled gas/liquid discharge of the heat exchanger. The temperature sensing element of the IR detector or guidance system is mounted in heat conducting relation with the reservoir. After the liquid cryogen is collected, the fluid flow through the heat exchanger is stopped and the collected cryogen is vented to a vacuum or other source of reduced pressure so that further heat is transferred from the temperature sensing element to the cryogen as the cryogen evaporates or sublimes, even to the point where the cryogen freezes and cools the temperature sensing element below the triple point of the cryogen, e.g., 63 degrees Kelvin for Nitrogen.
One such cryostat is shown in my U.S. Pat. No. 5,012,650 in the form of a Joule Thomson heat exchanger directly connected to a cryogen reservoir containing a composite matrix material particularly useful in retaining an inventory of liquid cryogen in the reservoir derived from the gas/liquid discharge of the heat exchanger as well as in retaining a significant amount of the liquid/solid cryogen when it is subsequently cooled by reducing its pressure. In an improvement of this cryostat, which has been described and claimed in my U.S. patent application entitled "Method & Apparatus for Collecting Liquid Cryogen", filed concurrently herewith, the gas/liquid discharge from the heat exchanger is directed to flow through this matrix material within the reservoir in order to fill the reservoir more rapidly.
An important feature of the cryostat shown in this U.S. Pat. No. 5,012,650 is that the reservoir is also directly connected to a venting conduit in the form of a large diameter vent tube through which the gas from the reservoir can vent to the vacuum. Preferably, this vent tube also forms the core of the heat exchanger. Without this venting conduit, the cryogen within the reservoir would be forced to vent through the high impedance of the heat exchanger itself. Since the temperature at which a cryogen evaporates or sublimes is a function of its pressure, the very low impedance of the large diameter venting tube allows the cryogen more rapidly to achieve lower pressures and to reach lower temperatures.
However, during the initial period that the gas is flowing through the heat exchanger and into the reservoir, it is imperative that no flow of this gas out of the reservoir though this venting conduit occurs. Any such venting flow will by-pass the effect of the heat exchanger and reduce its cooling power during this reservoir filling stage. This is especially true if the reservoir venting conduit also forms the core of the heat exchanger. It is also imperative that the venting conduit be fully opened shortly after the fluid flow within the heat exchanger is stopped. Any significant delay will allow the liquid cryogen in the reservoir to rapidly warm up, and any partial opening will inhibit the evaporation and sublimation of the cryogen, thereby reducing the rapidity and effectiveness of the cool-down.